Allow authentication for external VCS source_link endpoint

[bruno-garcia]

Sentry is able to pull source context into the stack trace when processing events.

Context:
.NET has the concept of source_link where the compilation process tags the build output (exe/dll) with the VCS information: URL and SHA.
Symbolicator leverages this data to fetch the source code of stack traces and populate the source context. This works great for .NET itself, since it's open source and on GitHub: For example:

Links to: https://github.com/dotnet/aspnetcore/blob/c2a442982e736e17ae6bcadbfd8ccba278ee1be6/src/Servers/Kestrel/Core/src/Internal/Http/Http1ChunkedEncodingMessageBody.cs#L109

This issue tries to expand this functionality to private repositories, by allowing the user to set the authentication to those requests.
One way to do this is to piggyback on our integrations with VCS. Customers on Azure DevOps for example who already have an integration with Sentry, already have 'auth' set up.

Relates to:

• Mistaken download of HTML file from dev.azure.com during C# symbolication #1457